Diamond/carbon/carbon composite useful as an integral dielectric heat sink

ABSTRACT

A diamond/carbon/carbon composite is provided comprising a carbon/carbon composite having a polycrystalline diamond film deposited thereon. The carbon/carbon composite comprises a preform of interwoven carbon fibers comprising vapor grown carbon fibers. The preferred method of producing the composite involves chemical vapor infiltration of the pyrolytic carbon into the interstices of the preform, followed by microwave plasma enhanced chemical vapor deposition of the diamond film on the carbon/carbon composite. The resulting diamond/carbon/carbon composite is useful as an integral dielectric heat sink for electronic systems in spacecraft, aircraft and supercomputers due to its thermal management properties. Such a heat sink can be made by depositing metallic circuits on the diamond layer of the diamond/carbon/carbon composite.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.F33615-92-C-2240 awarded by the U.S. Air Force Systems Command. TheGovernment has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 08/044,223, filed Apr. 7,1993, now U.S. Pat. No. 5,389,400.

BACKGROUND OF THE INVENTION

The present invention relates to a unique diamond/carbon/carboncomposite useful as an integral dielectric heat sink, and a method formaking such a composite. More particularly, the present inventionrelates to a diamond/carbon/carbon composite formed by depositingpolycrystalline diamond on a carbon/carbon composite, and to an integraldielectric heat sink formed by depositing a metallic film on the diamondlayer of the resulting diamond/carbon/carbon composite.

In recent years, demand has increased for high power, high densityelectronic devices used for advanced systems such as aircraft,spacecraft, and supercomputers. One known solution to this problementails fabricating multichip module (MCM) circuitry with greatercircuit density and more efficient electrical performance. However,while this process provides superior electrical performance, theincreased circuit density raises the power density within the MCM,increasing the heat dissipation requirements. Ongoing improvements toheat sink materials must be made to avoid heat-induced failure of suchelectronic devices.

Semiconductor bases have been produced using composites comprisingfibers, fiber bundles, and woven fiber bundles of graphite, boron,tungsten and glass. However, such composites do not have a very highthermal conductivity.

Carbon fiber reinforcements have made significant improvements inproperties of composites of various polymeric, metal, and ceramicmatrices. However, a limiting factor in the use of such fibers is theirhighly anisotropic thermal conductivity and their inability to match thecoefficient of thermal expansion of the circuit material, which istypically silicon or gallium arsenide.

Various processes have been developed in recent years to producesynthetic diamonds for use as heat sinks in the electronics field.Although diamond is electrically insulating, diamond has the highestthermal conductivity found in nature. Thus, diamond is an ideal heatsink material for semiconductor devices. Diamond has other desirableproperties including high hardness, high dielectric strength andbreakdown fields, chemical stability, and a coefficient of thermalexpansion (CTE) which is suitable for matching a wide range of CTE's ofmaterials to which the composite is mated.

One process for synthesizing diamond is to use microwave assistedchemical vapor deposition to deposit diamond on silicon substrates. Sucha process is described in Badzian et al, "Vapor Deposition Synthesis ofDiamond Films", Proceedings of SPIE, Vol. 683, 1986. Another processinvolves growing diamond crystals by plasma chemical vapor deposition asdescribed in Chang et al, "Diamond Crystal Growth by Plasma ChemicalVapor Deposition", J. Appl. Phys. 63(5), 1988.

Still other processes have been developed for production of layers ofpolycrystalline diamond on a substrate with recovery of flakes for usein composites. One such process is described in Banks, U.S. Pat. No.4,437,962. A composite is produced by depositing carbon on a surface,creating diamond bonds in the carbon, removing flakes of the carbon, andmixing the flakes with a matrix material to form the composite material.However, the production of such flakes requires a relatively complexprocess.

Accordingly, there is still a need in the art for an improved compositehaving superior electrical and thermal performance, low density and highmechanical strength. Further, there is a need for an improved compositewhich may be used as a high thermal conductivity dielectric heat sink.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a uniquediamond/carbon/carbon composite having a polycrystalline diamond filmdeposited on a carbon/carbon composite which exhibits superiorelectrical and thermal performance, low density, and high mechanicalstrength. The present invention further provides an innovative integraldielectric substrate or heat sink material useful as a semiconductorbase.

In accordance with one embodiment of the invention, adiamond/carbon/carbon composite is provided comprising a carbon/carboncomposite and a polycrystalline diamond film deposited on thecarbon/carbon composite. In a preferred embodiment of the invention, thecarbon/carbon composite comprises a preform of interwoven carbon fibershaving pyrolytic carbon deposited into interstices of the preform.

The carbon fibers preferred for use in the present invention aregraphitic fibers. Preferably, the graphitic fibers are vapor growncarbon fibers, which have a very high thermal conductivity. Such fibershave also been called benzene derived fibers and catalytic chemicalvapor deposition fibers. However, for purposes of the present invention,the fibers will be referred to as vapor grown carbon fibers. Vapor growncarbon fibers have a tensile strength as high as 7 GPa, as well as atensile modulus of 600 Gpa. In addition, the room-temperature thermalconductivity of vapor grown carbon fibers is in the range of 2000 W/m-K.

A method of making the diamond/carbon/carbon composite is also providedwhich generally comprises the steps of densifying a preform ofinterwoven carbon fibers by depositing pyrolytic carbon into theinterstices of the preform to produce a carbon/carbon composite. Apolycrystalline diamond film is then deposited on the carbon/carboncomposite.

Preferably, the preform is densified by a chemical vapor infiltrationprocess which uses a mixture of hydrogen and hydrocarbon gases. Thehydrocarbon gas preferably comprises at least 50% but not more than 75%by volume of the mixture. The chemical vapor infiltration process ispreferably carried out at a temperature of from about 1000° C. to about1100° C., and at a pressure of from about 5 torr to about 40 torr.

In an alternative embodiment of the invention, the preform is densifiedby a pitch infiltration process in which molten pitch from coal tar orpetroleum sources is impregnated into the fiber preform under heat andpressure. This process is followed by pyrolysis and subsequentre-impregnations until the pores between the fibers are filled to thepoint of sealing, all as is known in the art. See, for example, U.S.Pat. Nos. 4,490,201 and 4,396,663, the disclosures of which are herebyincorporated by reference.

The polycrystalline diamond film is preferably deposited on theresulting carbon/carbon composite by a microwave plasma enhancedchemical vapor deposition process using a mixture of hydrogen andhydrocarbon gases. The preferred hydrocarbon gas is methane, whichpreferably comprises at least 0.1% but not more than 2% by volume of themixture. The process is preferably carried out at a temperature range offrom about 850° C. to about 1100° C., and a pressure of from about 15torr to 50 torr. When deposited on the surface of the carbon/carboncomposite, the diamond becomes partially infiltrated into thecarbon/carbon composites. By infiltrated, we mean that the pyrolyzedcarbon near the surface of the carbon/carbon composite is partiallyreplaced by polycrystalline diamond.

In an alternative embodiment, the diamond film may be deposited by a hotfilament technique. This process is preferably carried out at atemperature range of from about 850° C. to about 1100° C., and apressure of from about 15 torr to 50 torr.

The resulting diamond/carbon/carbon composite may be used as a highperformance integral dielectric heat sink or substrate material havinghigh thermal conductivity, and low electrical conductivity. In addition,the diamond carbon/carbon composite has a coefficient of thermalexpansion which may be tailored to improve reliability and high thermalenergy management of electronic devices including electronics for spaceand aerospace vehicles. For example, in one embodiment of the invention,the diamond/carbon/carbon composite may be further formed into anintegral dielectric heat sink by depositing a metallic film on thediamond layer of the diamond/carbon/carbon composite.

Accordingly, it is an object of the present invention to provide adiamond carbon/carbon composite having high thermal conductivity,electrical insulation, and low density. It is also an object of thepresent invention to provide a diamond carbon/carbon composite which isuseful as an integral dielectric heat sink. This, and other objects andadvantages of the invention will become apparent from the followingdetailed description, the accompanying drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the diamond/carbon/carbon compositeof the present invention; and

FIG. 2 is a cross-sectional view of an integral dielectric heat sinkhaving the diamond/carbon/carbon composite as its base.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The diamond/carbon/carbon composite of the present invention is capableof advancing the performance capacity of power electronics. Thecarbon/carbon composite has a higher thermal conductivity and lowerdensity than other materials such as metals and ceramic materialsconventionally used in electronic packaging. The carbon/carbon compositeis also an electrical conductor, which typically requires anelectrical-insulating surface layer for such electronic packagingapplications. However, we have found that by depositing a diamond filmon the high thermal conductivity carbon/carbon composite of the presentinvention, electrical insulation is achieved without the need forconventional electrically insulating coatings. The application of thediamond film on the carbon/carbon composite also facilitates electricalcircuit printing directly on the diamond/carbon/carbon composite.Further, because the diamond coating partially infiltrates the surfaceof the carbon/carbon composite, the coefficient of thermal expansion ofthe resulting diamond/carbon/carbon composite may be tailored forspecific applications.

The diamond/carbon/carbon composite 10 of the present invention isillustrated in FIG. 1. The diamond/carbon/carbon composite comprises acarbon/carbon composite 12 comprised of a preform of interwoven carbonfibers 16. A polycrystalline diamond film 14 is deposited on thecarbon/carbon composite. As shown in the drawings, the diamond coatingpartially infiltrates the surface of the carbon/carbon composite to acertain depth which for representation purposes is shown as beingslightly more than one row of carbon fibers deep.

The carbon fibers 16 preferably comprise graphitic fibers which arevapor grown carbon fibers. Vapor grown carbon fibers are preferred asthey have a tensile strength as high as 7 GPa, as well as a high tensilemodulus of greater than 480 GPa. The vapor grown carbon fibers have aroom-temperature thermal conductivity in the range of about 2000 W/m-K,which is several times that of copper and aluminum, and essentiallyequals that of single crystal graphite. Such fibers have a graphiticnature which resembles an "onion-ring" structure of graphene planeswithin the fibers.

Densification of the vapor grown carbon fiber preform with pyrolyticcarbon prior to diamond deposition may be accomplished by conventionalmethods including chemical vapor infiltration and pitch infiltration.Preferred is a chemical vapor infiltration process which involves thepyrolysis of a hydrocarbon gas and deposition of pyrolytic carbon intothe preform. The formation of pyrolytic carbon is characterized bydevelopment of basal graphitic planes in the same method orientation asthe carbon fibers. A gaseous mixture of a hydrocarbon gas and hydrogenis used for this purpose. A preferred hydrocarbon gas is methane, whichpreferably comprises at least 50% but not more than 75% by volume of themixture.

The chemical vapor infiltration process should be carried out at atemperature of from about 1000° C. to about 1100° C., and the pressureshould be maintained at a range of from about 5 torr to about 40 torr.However, it should be noted that the extent of densification may becontrolled by varying the concentration of the hydrocarbon gas alongwith pressure, temperature, and time parameters. The pyrolytic carbon inthe resulting carbon/carbon composite is highly aligned and closelyresembles single crystal graphite.

An alternative method of densification is pitch infiltration. In thisprocess, molten pitch from coal tar or petroleum processes isimpregnated into the fiber structure, followed by pyrolysis andsubsequent re-impregnations until the pores between the fibers arefilled to the point of sealing as disclosed in U.S. Pat. Nos. 4,490,201and 4,396,663. This process results in an aligned, graphiticmicrostructure.

The deposition of the polycrystalline diamond 14 onto the carbon/carboncomposite 12 is preferably accomplished by a microwave plasma enhancedchemical vapor deposition using a mixture of hydrogen and hydrocarbongases. The preferred hydrocarbon gas is methane, which preferablycomprises at least 0.1% but not more than 2% by volume of the mixture.In order to achieve uniformity of the diamond coating, the process ispreferably carried out at a temperature range of from about 850° C. toabout 1100° C. and at a pressure of from about 15 torr to 50 torr.

Alternatively, the polycrystalline diamond may be deposited by a hotfilament technique, which is preferably carried out at a temperaturerange of from about 850° C. to about 1100° C. and at a pressure of fromabout 15 torr to 50 torr.

The resulting diamond/carbon/carbon composite is especially suited foruse as an integral dielectric heat sink. As shown in FIG. 2, a metalliclayer 18 of the desired circuit pattern is deposited on the diamondlayer 14 of the diamond/carbon/carbon composite. Metallization of thediamond/carbon/carbon composite may be achieved by sputter depositing alayer of titanium or molybdenum in a range from about 200 angstroms toabout 500 angstroms on the diamond/carbon/carbon composite. This step isfollowed by high temperature annealing to create a good chemical bondbetween the diamond and the metal layer. Next, a layer of copper, gold,platinum or gold/nickel is deposited onto the metal layer on thediamond/carbon/carbon substrate by electro-plating to achieve the filmthickness that is required for proper current carrying capabilities. Theresulting product is a diamond/carbon/carbon composite 20 upon whichsemiconductors can be mounted using conventional surface-mounttechnology.

Alternatively, metallization may involve spreading photoresist, a lightsensitive emulsion, uniformly across the prepared composite. A mask witha desired trace pattern is then placed over the photoresist and exposedto light. The composite is then rinsed with a developing solution thatwashes away the exposed photoresist. A metallic layer is then depositedonto the composite. Finally, the unexposed layer of photoresist isdissolved and washed away by a solvent solution.

Another method of making a dielectric heat sink material is to lay ametallic layer on the diamond/carbon/carbon composite, followed by theapplication of the photoresist on top of the metallic layer. A mask forthe traces is then applied on top of the photoresist. The exposedphotoresist is then washed away, followed by the etching away of theunwanted portion of the metallic layer using an acidic solution.

The diamond/carbon/carbon composite has considerable potential in futureelectronic devices. The diamond film allows electric circuit printing tobe successfully applied directly upon the composite material.Additionally, the diamond film provides electrical insulation from thecomposite material, and a high thermal conductivity medium into thecomposite material, thus preventing heat-induced failures in electronicdevices. Thermal conductivity perpendicular to the z plane of the heatsink is maximized by the use of the diamond film while high thermalconductivity in the plane of the heat sink is obtained due to theproperties of the carbon/carbon composite.

The following example is for purposes of illustrating the presentinvention. The example is not to be considered as limiting the scope ofthe claims appended hereto.

EXAMPLE 1

A diamond/carbon/carbon composite in accordance with the presentinvention was prepared. First, vapor grown carbon fibers were producedby the pyrolysis of a hydrocarbon gas in the presence of a catalyst in atwo-stage process. Small particles of iron, with diameters less than 10nm, were spread on a substrate as a catalyst for fiber growth. Thesubstrate was then positioned in a furnace in which the temperature wassubsequently ramped from room temperature to 900° C. At thistemperature, a mixture of methane and hydrogen was introduced toinitiate the fiber nucleation and growth. The filament diameter was thenenlarged during a second stage by chemical vapor deposition of carbonusing increased methane concentration. Vapor grown carbon fiber preformswere then fabricated by lay-up of vapor grown carbon fibers which werecut to the dimensions of a mold, formed into prepregs, and then cured ina hot-press. The preforms were slowly carbonized at 900° C. over a threeday period. Following the carbonization, the preforms were graphitizedat 2800° C. prior to densification.

Densification of the vapor grown carbon fiber preforms was achievedusing a chemical vapor infiltration technique at a temperature between1020° C. to 1080° C. and a pressure of 15 torr. The gaseous mixture usedcomprised 75% methane and 25% hydrogen. The composites were then heattreated at 2800° C. in argon for 15 minutes to graphitize the carbon.

The resulting carbon/carbon composites were then deposited with diamondusing a microwave enhanced plasma chemical vapor deposition technique.Raman analysis of the composites was performed, and the Raman spectra ofdeposited diamond indicated high quality diamond formation at thesurface of the carbon/carbon composite. Electrical conductivitymeasurements were also carried out on a specimen deposited with diamond.The conductivity was shown to be substantially higher than naturaldiamond, but still sufficiently low to be suitable as a dielectriclayer.

EXAMPLE 2

Two samples of carbon/carbon composites were fabricated in accordancewith the present invention. The specifications of the samples are shownbelow in Table I.

                  TABLE I                                                         ______________________________________                                                                       Specific                                             FV     Density  Conductivity                                                                           Conductivity                                                                           CTE                                   Sample                                                                              (%)    (g/cc)   (mK)     (W/mk/g/cc)                                                                            (ppm/K)                               ______________________________________                                        1     28     1.27     X: 364   287      0.12                                  2     28     1.52     X: 372   245      0.41                                                        Z: 15                                                   ______________________________________                                    

Diamond was deposited on the carbon/carbon composite samples using amicrowave system at gaseous mixtures of 99.9% H₂ /0.01% CH₄ and 99.5% H₂/0.5% CH₄. Following deposition, it was observed that the pyrolyticcarbon on the surface of the carbon/carbon composite was etched away andpartially replaced by polycrystalline diamond. Raman analysis wascarried out on the sample containing 0.5% CH₄, and the resultingspectrum showed a diamond peak at 1333.1 cm⁻¹ and a graphite peak at1439.2 cm⁻¹.

Deposition of diamond on Samples 1 and 2 was also carried out using thehot filament technique. Samples were deposited with diamond for 9 and 33hours, respectively. Raman analysis of the two samples was thenperformed. The Raman spectrum for the 9-hour sample showed a diamondpeak at 1331.9 cm⁻¹ and a broad background spectral feature indicating asmall contribution from the graphite in the Raman spectrum of thesample. The sample exposed for 33 hours showed that the middle region ofthe sample received thicker deposition. The Raman spectrum for thissample demonstrated a single peak of diamond located at 1332.2 cm⁻¹. Nocontribution to the Raman spectrum from graphite was observed.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. A diamond/carbon/carbon composite comprising:acarbon/carbon composite comprising a preform of interwoven vapor growncarbon fibers having interstices therein and having pyrolytic carbondeposited into interstices of said preform; and a polycrystallinediamond coating directly on said carbon/carbon composite forming apolycrystalline diamond film, whereby said polycrystalline diamondcoating partially infiltrates the surface of said carbon/carboncomposite.
 2. The composite of claim 1 further including a metalliclayer deposited on the diamond layer of said diamond/carbon/carboncomposite.
 3. The composite of claim 1 wherein said vapor grown carbonfibers have a room-temperature thermal conductivity of about 2000 W/m-k.4. The composite of claim 2 further including a layer selected from thegroup consisting of copper, gold, platinum and gold/nickel deposited onsaid metallic layer.
 5. The composite of claim 2 wherein said metalliclayer is selected from the group consisting of titanium and molybdenum.